Humpback whale adult females and calves balance acoustic contact with vocal crypsis during periods of increased separation

Abstract

Acoustic communication is important for animals with dependent young, particularly when they are spatially separated. Maternal humpback whales (Megaptera novaeangliae) use acoustic calling to help minimize the risk of separation from their young calves during migration. These pairs also use acoustic crypsis to minimize detection by males. How they balance a restricted active space with the need to maintain acoustic contact during periods of separation is not yet understood. Here, we analyzed movement metrics of tagged adult female-calf pairs during migration to identify two behavioral states, "resting/milling" and "travelling." When travelling, these pairs dived synchronously and exhibited little to no spatial separation. Alternatively, adult females had significantly longer dive durations (p < .01) when resting, and while they spent prolonged times at depth, calves would surface several times independently. This demonstrated that these pairs are frequently separated during periods of rest. We then determined whether the call rates and acoustic levels of these pairs increased with more frequent separation, finding that both adult females and calves significantly increased their call rates, but not levels, when resting. We also found that adult female-calf pairs have a restricted active space, with less than 15% of calls estimated to be detectable beyond 2 km. However, as with call level, detection distance did not differ significantly between the two behavioral states. In summary, adult female-calf pairs maintain successful communication during periods of separation by calling more frequently rather than by producing louder calls. This strategy aids in maintaining acoustic contact while simultaneously limiting detectability by conspecifics.

Comparing the social signaling behavior of humpback whales in three group types on the Hawaiian breeding grounds using acoustic tags

“…In contrast to competitive groups, escorted mother-calf pairs produced calls less frequently with longer inter-call intervals. The longer ICIs observed in escorted mother-calf pairs were somewhat expected and are likely explained by known mother-calf calling behavior (Indeck et al, 2022) and the group interactions observed during the tag deployments. Although escorts show a preference for females without a calf, as they typically have a greater reproductive potential than maternal females (Craig et al, 2003), they will also prospect among mothers for those that may be in post-partum estrus (Chittleborough, 1958;Pallin et al, 2018).…”

“…The addition of one or more escorts to a mother-calf pair increases their energy expenditure (Craig et al, 2014) and could result in injuries to the calf or its separation from its mother (Smultea, 1994). As such, these pairs must balance communication with each other with the potential of being joined by escorts, which is believed to be reflected by lower call rates and levels in these groups (Videsen et al, 2017;Indeck et al, 2022). Nevertheless, these pairs have been found to increase calling rates during periods of increased separation as a means of maintaining acoustic contact while continuing to limit detectability by nearby whales (Indeck et al, 2022).…”

“…As such, these pairs must balance communication with each other with the potential of being joined by escorts, which is believed to be reflected by lower call rates and levels in these groups (Videsen et al, 2017;Indeck et al, 2022). Nevertheless, these pairs have been found to increase calling rates during periods of increased separation as a means of maintaining acoustic contact while continuing to limit detectability by nearby whales (Indeck et al, 2022). Similar calling behavior is likely also useful during nonagonistic interactions with a single escort, as a means of minimizing the risk of mother-calf separation, as well as reducing the possibility of attracting additional escorts.…”

“…However, the affiliation between group members may also be a factor that contributes to differences in call rates among groups. For example, solitary mother-calf pairs have likely evolved lower call rates as a strategy to avoid detection by predators and harassment from males (Craig et al, 2014;Dunlop, 2016;Videsen et al, 2017;Indeck et al, 2021;Indeck et al, 2022). Alternatively, several studies have shown that humpback whales increase their calling rate when unaffiliated whales join the group (Silber, 1986;Rekdahl et al, 2015;Cusano et al, 2020), and it has been proposed that variable call rates across escorted groups are attributable to the presence of males and differences in behavioral context (Seger, 2016, dissertation).…”

Pre- and Postpartum Acoustic Activity in Captive Pacific White-Sided Dolphin (Lagenorhynchus obliquidens) Mothers

“…The mother–calf pairs of humpback whales ( Megaptera novaeangliae ) and southern right whales ( Eubalaena australis ) infrequently produce calls during diving associated with nursing (Videsen et al 2017; Nielsen et al 2019). Mother–calf pairs also use low-amplitude calls to restrict their acoustic active space (Videsen et al 2017; Nielsen et al 2019; Parks et al 2019), even if they increase the call rate during more frequent separation (Indeck et al 2022). The dominant energy of pulsed call sequences produced by Pacific white-sided dolphins is below 100 kHz and can be easily detected by killer whales, which are one of their predators (Black 2018).…”